Finely divided metal salts as antioxidants for oils and greases



7 3,000,819 FINELY DEVEDEDMETAL SALTS AS ANTI- FOR OILS AND GREASES 7 James H. Norton, Corunna, Ontario, and Lorne W.

Sproul e, Sarnia, Qntario, Canada, assignors to; Esso Research and Engineering Company, a corporation of Delaware No Drawing. FiledJ'an. 11, 1 954, Ser. No. 403,432

6 Claims. (Cl. 252-45) The present invention relates to an improved method of inhibiting the oxidation of oils and greases, especially mineral lubricating oils and greases. More particularly. the invention relates to an' improved method of dispersing an oxidation inhibiting dehydrated metal salt in oils and greases. In its broadest aspect, the invention pertains to the dispersion in oils and greases, of metal salts having oxidation inhibiting activity by introducing crystals of a hydrated form of the metal salts into oil heated to a temperature substantially above that at which the hydrated of the metal salt decomposes.

The use of various inorganic salts as" oxidation inhibitors for mineral oil products, particularly lubricating oils and greases, has been suggested heretofore. Certain polymetal phosphates, e.g. trisodium phosphate are outstanding examples of these inhibitors. These phosphates are insoluble in hydrocarbon oils. However, they exert their oxidation inhibiting eltect even when contacted" with the oil in the undissolved solid state.

According to the present invention the oxidation inhibiting salt such as a poly-metal phosphate, particularly trisodium phosphate is added to the hydrocarbon or other material to be treated, in the form of ultra-fine solid particles obtained by introducing crystals of a hydrate of the salt into a liquid maintained at a temperature substantially above the dehydrating temperature of the salt hydrate. Temperatures about 50-400 F., preferably about 100- 300 -F. above that at which dehydration occurs are desirable. These temperatures may fall within the range of about 250-600 F. When so' operating, the salt hydrate crystals are dehydrated with almost explosive speed and force, the generated steam disrupting the crystal structure and disintegrating the crystals to particles of extremely small size comparable to that obtained by adding the salt in the form of an aqueous solution. The liquid used to dehydrate and disintegrate the salt hydrate may either be the material to be treated or any other suitable .liquid in which the salt is insoluble, such as a mineral oil. In the latter case, the suspension formed may be added as suchto the material to be treated. As the result of using the salt in the undissolved solidstate, no-- complex withany soap present inthe process material is formed and no undesirable plasticizat-ion of such soap is encountered.

The metal salts used as oxidation inhibitors in accordance with the present invention should be mildly alkaline. The choice of a specific salt depends to a certain extent on the temperature range for which stabilization is desired. In general, the sodium and potassium phosphates and silicates containing a plurality of metal atoms per molecule and crystallizing with at least 2 moles, preferably about 6-12 moles, of water of crystallization are most suitable, particularly for high temperature service. Salts of substantially weaker alkalinity, such as alkali metal salts of organic acids, nitrites, etc., are relatively ineffective.

Hydrated trisodium phosphate having 8-12 moles of water of crystallization is the preferred hydrated metal salt used in accordance with the invention. Hydrated tripotassium phosphate has similar oxidation inhibitingp'roperties, but normally contains only 8' moles of water of nite States Patent the invention.

hhhhflih Patented Sept. 19, 196i crystallization. Slightly higher oil temperatures may, therefore, be necessitated in the mixing process in order to obtain upon dehydration of the salt a disintegration effect comparable to that obtainable with Na PO .I2H O. Examples of other salts suitable for the purposes of the invention, though with markedly lower efiiciency, include hydrated di-alkali metal phosphates such as Na4P207. 1 etc.

These salts may be used in proportions as low as 0.1% by weight of the hydrocarbon material to be treated, calculated as dehydrated salt. Proportions in excess of 0.25% are desirable and amounts of about 0.5-5 are preferred on this basis. Quantities as high as 20% of these inorganic metal salts can, however, be incorported into the grease, if desired.

The temperature of the medium to which the salt crystals are added has a direct bearing on the particle size of the oxidation inhibitor in this material and thus on the oxidation inhibiting effect. Generally, the higher this temperature the smaller is the particle size obtained. For example, in the case of hydrated trisodium phosphate the average particle size may be about 20 microns (0.02 mm.) or larger when the oil temperature is in the neighborhood of 300 or 320 F., but may fall to about 5 microns (0.005 him.) when the temperature is raised to, say, about 400 or 420 F. Hydrated salts containing relatively little Water of crystallization, say about 2-5 moles, usually require somewhat higher temperatures, say about 50100 higher, for a desired particle size than salts havinga relatively high content of water of crystallization, such as 6-12 moles.

Within reasonable limits, the original particle size of the crystallized salt hydrates introduced into the hot hydrocarbon material has little, if any, influence on the ultimate particle size of the dehydrated salt in the hydrocarbon material. Thus, suitable original particle sizes of the hydrates vary within the wide limits of, say about 0.1 mm. microns) to about 3.0 mm. (3000 microns). Substantially coarser materials require a slight upward adjustmentof the dehydration temperature by, say, about 100 F.-

The material to be stabilized against oxidation inaccordance' with the present invention may be any oil in which the hydrated salt inhibitor is substantially insoluble. Hydrocarbon oils, particularly mineral oils of lubricating oil grade having viscosities ranging from about 50' SSU at 100 F. to' about 1,000 SSU at 210 F., are outstanding examples of such oils. Synthetic oils, such as di-(2- ethylhexyl) sebacate or similar ester oils of lubricating glrlade may be used in combination with suitable mineral o s.

This invention has particular utility in the stabilization of lubricating greases against oxidation. Many types of lubricating greases may be improved by the process of These greases include those prepared by thickening the lubricating oils mentioned above with conventional grease thickeners such as soap, complex soap and inorganic thickeners. Soap thickeners normally consist of the alkali metal, alkaline earth metal and/or aluminum soaps of high molecular weight fatty or hydroxy-fatty acids having 12-30, preferably 14-22 carbon atoms per molecule. Stearic acid, hydrogenated fish oil or tallow acids, mono, di, tri and tetra hydroxy stearic acids, oleic acid and ricinoleic acid are most commonly used for this purpose. Complex soap thickeners are formed by combining soaps of the type mentioned with about 0.3-3 moles of the alkali or alkaline earth metalsalts of low molecular weight carboxylic acids having about 1-6 carbon atoms, such as formic, acetic, lactic and similar acids. Inorganic grease thickeners include silica gel, amorphous silica, various natural and synthetic clays, carbon black, etc.

The invention aifords greatest advantages when applied to the treatment of soda soapor complex soap-thickened greases, because of the fact that oxidation inhibition may be achieved without detrimental effect on the softening characteristics of the grease. Also, the preparation of these greases normally involves a dehydration stage in which extraneous and reaction water is removed. This stage is usually carried out at temperatures well within the range required for the instantaneous dehydration of the hydrated salts of the invention. By adding these salts during or after the grease dehydrating stage and before cooling, no special heating step is required to carry out the process of the invention.

The process of the invention is best carried out by maintaining the material to be treated in the liquid state at a temperature of about 250-600 F. and introducing the solid finely divided hydrated salt into the hot material in a continuous stream or in small increments until the above-mentioned concentration is reached. Continuous agitation of the hot material while the salt is being added is desirable to assure uniform distribution of the dehydrated salt throughout the liquid to be treated. When applying the invention to grease, an oil concentrate of the dehydrated salt may be first formed by slowly introducing the hydrated salt as described into a hot oil so as to cause flash dehydration of the hydrated salt and adding the suspension formed, which may contain about l-50% by weight of salt, to the finished grease.

The invention and its advantages will be best understood by reference to the comparative experiments described hereinafter.

A grease A was prepared in the conventional manner at 300 F. from mineral lubricating oil containing 24.0 wt. percent of hydroxystearic acid and 3.3 Wt. percent of sodium hydroxide.

A grease B was prepared by adding about 16.0% of an aqueous solution of trisodium phosphate hydrate (Na PO .8H O) (26.0% dry salt concentration) to a grease batch of lubricating oil containing 20.0% of l2-hydroxystearic acid and 2.8% sodium hydroxide. The batch was then dehydrated to 300 F. in the conventional in an oven at 300 F. for 24 hours. To prepare the ball milled phosphate suspension the anhydrous phosphate (100 grns.) and mineral oil (200 gms.) were charged to the ball mill and the mill was rotated for 16 hours. It was found that the particle size of the phosphate was in the order of 5 microns (0.005 mm.). While the particle size no doubt could be further reduced by longer milling, this particle size of the phosphate is satisfactory. Greases prepared using this material have satisfactory oxidation characteristics as shown in the attached table.

, However, ball milling suffers from the disadvantage of being time consuming and that the cheaper phosphate hydrate must be dried before milling.

In a still further experiment, trisodium phosphate hydrate (Na PO .8H O) having a particle size of about 210 microns was added slowly to two batches of mineral lubricating oil maintained at 300 F. and 400 F., respectively, to form suspensions containing about 20% of anhydrous phosphate. These suspensions were worked into separate batches of grease A to introduce about 4 wt. percent of dehydrated phosphate into the finished grease. The average phosphate particle size at 300 F. was about 5-15 microns (0005-0015 mm.), that at 400 F. about 5 microns (0.005 mm.). These greases are designated as grease E and F, respectively. All greases described above were tested for their micropenetration and hardening tendency.

The conditions and results of these experiments are summarized in the table below. The grease hardening test employed is fullydescribed in a copending Sproule et a1. application, Serial No. 230,415, filed June 7, 1951, now abandoned, which is here referrred to for specific manner. A soap-phosphate complex was formed. details.

Oxidation reszstance of greases contammg anhydrous trlsodzum phosphate Designation of Grease A B O D E F Formula, Wt. Percent:

12-Hydroxystearic Acid-.- Sodium Sulfonate Sodium Hydroxide, Dry Trisodium Phosphate,

Dry. Mineral Oil (1) Phenyl-a-Naphthylamine (1) phenol extracted, MPK dewaxed, Vis. 100 F.; 550 SSU; V1. 90. Preparation of Trisodium Nil Soap-Salt Gom- Aqueous N33]? 0 80- Ball Milled Anhy- Na P 0 .8H2O hy- NasPO4.8H2O hy- Phosphate Slurries. plcx Prepared lution Slowly drous Nil-31 04 drate slowly drate slowly added at 300 F. Dripped into 100 Added to Oil added to 011 Base to Oil Base (1) at gins. of Oil Base (1) Base (1) and (1) at 300 F. 400 F. at 300 F. all Milled. Average Particle Size of An- -70 Mierons 5Microns, Quite 5-15 Microns 5 Mlcrons, Quite hlydtrous Trisodiurn Phosorm. Uniform. p a e. Inspections of Greases:

MFeropenetration, 77 55 661gkTM Pen. 66 63 85 77.

0 Grease Hardening Test, 1, 000 2,320 2,100 2,100 2,100 1,760.

Hrs. to Beach 5 Micropenetration at 300 F.

In another experiment (grease C), 394 gms. of an 70 aqueous solution of trisodium phosphate hydrate 14.3% concentration dry salt basis) was allowed to drip slowly into hot mineral oil (100 grns.) with constant stirring of the oil. The suspension was added to grease A to give a trisodium phosphate concentration of 4.0 wt. percent;

It will be noted that greases E and F which were prepared in accordance with the invention are, regarding hardening tendency, far superior to conventional grease A which was free of trisodium phosphate. The greases of the invention compare well in this respect with greases C and-D which were prepared by using aqueous solutions of trisodium phosphate or dehydrated trisodium phosphate powder.

It will be understood that improved oils and greases prepared in accordance with the invention may contain other conventional modifiers, such as other anti-oxidants, e.g. phenyl alpha-naphthylamine; corrosion inhibitors, e.g. sorbitan esters of fatty acids; tack'mess agents, e.g. polyisobutylene of about 1,000 mol. wt.; viscosity index improvers, e.g. poly-isobutylene of about l5,00020,000 mol. wt.; detergents, e.g. calcium sulfonate, sodium sulfonate, etc.; extreme pressure agents, e.g. sulfurized sperm oil or chlorinated Wax; etc.

The invention is not limited to the specific figures of the foregoing examples. The relative proportions of the materials used and the reaction conditions may be varied within the limits indicated in the specification to obtain products of varying characteristics.

What is claimed is:

1. In the method of inhibiting an oil selected from the group consisting of mineral and synthetic ester oils of lubricating grade by dispersing therein an oxidation inhibiting mildly alkaline polyalkali-metal phosphate salt which is insoluble in said oil, an improved method of dispersing said salt which comprises slowly adding to said oil in a dry undissolved solid state a crystallized hydrate of said salt containing in the range of 6 to 12 moles of water of crystallization per mole of hydrate, at a temperature 50 to 400 F. higher than the dehydration temperature of said hydrate to dehydrate and disintegrate said hydrate.

2. The method of claim 1 wherein a concentrate of 1 to 50 wt. percent of said salt, based on dehydrated salt, is prepared in said oil.

3. Process of claim 1 wherein the final concentration of said salt is in the range of 0.1 to 20 wt. percent based on dehydrated salt.

4. The method of claim 1 wherein the final product is a grease containing, in addition, a soda soap grease thickener.

5. The method of claim 1 wherein said salt is trisodium phosphate having in the range of 8 through 12 moles of water of crystallization per molecule.

6. The method of preparing a lubricating grease which comprises slowly introducing crystallized trisodium phosphate in a dry undissolved solid state having in the range of 8 through 12 moles of water of crystallization per mole into a mineral lubricating oil while maintaining a temperature in the range of 250 to 600 F., forming thereby a suspension of dehydrated trisodium phosphate in said oil, and incorporating said oil containing said suspension into a soda soap grease, the final amount of trisodium phosphate in said grease being in the range of 0.5 to 5 wt. percent, based on dehydrated salt.

References Cited in the file of this patent UNITED STATES PATENTS 2,079,051 Sullivan et al. May 4, 1937 2,417,827 Jones Mar. 25, 1947 2,455,659 Duncan et al. Dec. 7, 1948 2,614,986 Beerbower et al Oct. 21, 1952 2,625,508 Stross Ian. 13, 1953 2,626,899 Abrams et al. Jan. 27, 1953 2,656,315 Eckert Oct. 20, 1953 2,748,081 Peterson et al May 29, 1956 FOREIGN PATENTS 706,555 Great Britain Mar. 31, 1954 

1. IN THE METHOD OF INHIBITING AN OIL SELECTED FROM THE GROUP CONSISTING OF MINERAL AND SYNTHETIC ESTER OILS OF LUBRICATING GRADE BY DISPERSING THEREIN AN OXIDATION IN HIBITING MILDLY ALKALINE POLYALKALI-METAL PHOSPHATE SALT WHICH IS INSOLUBLE IN SAID OIL, AN IMPROVED METHOD OF DISPERSING SAID SALT WHICH COMPRISES SLOWLY ADDING TO SAID OIL IN A DRY UNDISSOLVED SOLID STATE A CRYSTALLIZED HYDRATE OF SAID SALT CONTAINING IN THE RANGE OF 6 TO 12 MOLES OF WATER OF CRYSTALLIZATION PER MOLE OF HYDRATE, AT A TEMPERATURE 50 TO 400*F. HIGHER THAN THE DEHYDRATION TEMPERATURE OF SAID HYDRATE TO DEHYDRATE AND DISINTEGRATE SAID HYDRATE. 